This invention relates to base support or mounting assemblies for movable antenna, more particularly to stepping motor driven supports for the horizontal and vertical rotation of dish antenna for audio, video or data signals.
It is well known that a satellite antenna may be mounted on a support having relatively movable parts which allow the antenna to be aimed toward a particular satellite in geostationary orbit about the earth to collect signals relayed and/or transmitted from that satellite. A description of the general operation of dish antenna and the relationship thereof to orbiting satellites beaming signals to such antenna is contained in U.S. Pat. No. 4,617,572, issued Oct. 14, 1986, the disclosure of which is incorporated herein by reference thereto.
As described in U.S. Pat. No. 5,281,975, it is known to provide a base support for a dish antenna having manual adjusting means to adjust the position of the antenna along a predetermined plane to direct the antenna toward the "Clark belt" or "geostationary satellite belt". Thereafter, second adjustment means on the base support, including a reversible motor, may be used to scan back and forth along the satellite belt until desired signals from a particular satellite are clearly being received by the dish antenna.
According to U.S. Pat. 5,281,975, and as clearly shown in FIGS. 1-5 thereof, the reversible electric motor M drives a worm 62 which is in intermeshing contact of a worm gear 30. The motor M is mounted stationary in the horizontal plane, while revolution of worm 62 will cause worm gear 30 to move in a horizontal plane i.e. about a vertical axis, thereby repositioning the attached dish antenna.
In the aforestated systems, the worm is actuated by a DC motor driven by a power source. To move the assembly incrementally, the DC motor sends back a pulse count to a controller by means of a high to low voltage signal, such as a twelve (12) to 36 (thirty-six) volts being the high voltage end 0 (zero) to 1 (one) being low voltage. This change in voltage is a count pulse and is accomplished, generally, by opening and closing a switch in a fixed power supply source. The switch may be magnetic, mechanical, or solid state.
The controller operates until it receives a predetermined count and then shuts down. The count speed is predetermined by the rotational speed of the unit. The rotational speed of the unit is determined by the voltage and the load on the motor. This type of feedback control does not allow for particularly fine increments of movement. This is a problem on small or miniature antenna assemblies, for example mobile mounted antenna, which require fine positional adjustments relative to the change in position of the antenna-bearing mobile unit.
It will be appreciated that worm gear drives have two forces working in the system. There are axial forces along the axis of the drive worm and radial forces which tend to push the drive worm out of the worm gear. In a worm driven satellite antenna mount, zero backlash is required since the satellites are positioned at two degree (2.degree.) spacing. The movement of the antenna due to backlash between the worm and worm gear would cause a loss of signal strength which weakens the reception or can cause a complete loss of reception. For example, on a 30 tooth worm gear each tooth represents twelve degrees (12.degree.) of rotation. At a pitch diameter of 1.25 inches, one tooth is the circumference divided by 30 or 3.927 inches divided by 30 or 1.309 inches. One degree of rotation of the antenna is 0.1309 divided by 12 or 0.0109 inches. A 10% error of movement ins 0.0011 inches of movement on the circumference or 1% of a tooth. Such movement occurs when the worm drive moves away from the worm gear or by the worm drive moving axially.